Method for agglomerating fine-grain iron oxide material

ABSTRACT

A method for agglomerating fine grain iron oxide material, preferably magnetite, the material being agglomerated in a cold condition or when heated to temperatures up to 100* C, in which the material is agglomerated by being rolled between substantially smooth or grooved rolls in the presence of a lubricant which is added in such quantities that the friction between the grains of the material is substantially reduced and at most in quantities which can be vaporized by the heat developed by said friction.

United States Patent Giirling et al.

[4 1 Jan. 14,1975

METHOD FOR AGGLOMERATING FINE-GRAIN IRON OXIDE MATERIAL Inventors: Karl Giiran Giirling, Lidingo; Johan Eloi Wiklund, Halsingborg, both of Sweden Assignee: Boliden Aktiebolag, Stockholm,

Sweden Filed: Apr. 12, 1972 Appl. No.: 243,499

Related U.S. Application Data Continuation of Ser. No. 863,367, Oct. 2, 1969, abandoned.

Foreign Application Priority Data Oct. 16, 1968 Sweden 13972 U.S. Cl. 75/3 Int. Cl C2lb 1/20 Field of Search 75/863, 367, 3

References Cited UNITED STATES PATENTS Rouse 75/3 1,073,381 9/1913 Wolle et a1 75/3 1,087,183 2/1914 Ronay 2,346,034 4/1944 Kramer 2,789,894 4/1957 De Vaney 3,235,371 2/1966 Volin et a] 3,420,453 l/1969 Tada et al 75/3 X Primary Examiner-A. B. Curtis Attorney, Agent, or FirmStevens, Davis, Miller and Mosher [57] ABSTRACT 4 Claims, No Drawings METHOD FOR AGGLOMERATING FINE-GRAIN IRON OXIDE MATERIAL This is a continuation of application Ser. No. 863,367, filed Oct. 2, 1969, now abandoned.

The present invention relates to a method of agglomerating by rolling fine iron oxide material intended as raw material in the manufacture of iron and steel.

Fine material of this nature often presents nondesirable physical properties. For example they create dust clouds, have low volume weight and tend to take up large quantities of water when moistened, which renders the material less suitable for handling, transport and further processing, e.g. in a metallurgical process. Examples of such processes include sintering in a furnace or on a conveyor, also treatment in rotary furnaces, shaft furnaces, flame furnaces, electric smelt furnaces etc.

For the purpose of producing a coarser material from fine material suitable for suction sintering for example, attempts have been made to granulate the material by, for instance, micropelletizing methods. Pelletizing involves agglomerating the material while adding water and binding agents, and in the majority of cases presumes subsequent drying and/or combustion steps at elevated temperatures. Pelletizing in drums or on plates is a process which requires accurate supervision and is sensitive to variations in moisture, peripheral velocity and the physical composition of the material, particularly with regard to grain distribution. Consequently there are a large number of fine materials which it is impossible to pelletize without adjusting the grain distribution by employing expensive grinding operations. Fine grain materials of low volume weight which are prone to take up large quantities of water are particularly difficult to agglomerate when applying suction sintering processes if the material is not granulated prior to being sintered. To granulate the material by micropelletizing processes is particularly difficult. A typical example of a fine grain material of low volume weight which tends to absorb large quantities of water is iron oxide, removed from roasting of flotation pyrite concentrate.

In order to'solve the problems of agglomerating material not suited for suction sintering processes, the socalled pellet sintering process has been devized, according to which pellets which can be charged directly to blast furnaces are produced by pelletizing and burning. Briquetting, either in hot state in the absence of a binding agent or at lower temperatures in the presence of a binding agent has also been suggested and tested for the purpose of producing agglomerates from various fine grain iron raw materials, which can be charged direct to blast furnaces or as a refining material in steel manufacture. Two modifications for briquetting in hot condition have been developed. In accordance with one of said modifications, the material is heated to high temperatures, approximately 1 100C, prior to briquetting in briquette moulds mounted on rolls, and in the other modification the material is reduced before being briquetted, whereby the agglomeration temperature can be lowered by about 200C.

These hot briquetting methods, however, are accompanied with certain disadvantages. Heating of very fine materials to temperatures as high as 1 100C is difficult from a technical point of view and is also difficult to perform with good heat economy. The most difficult problem, however, is the wear of the briquette molds and no material has heretofore been found which causes so little wear on the moulds that the process becomes economic. In addition, a large quantity of the fine material is returned to the system. Neither are briquettes produced in this way suitable for all purpose. It is, for example, difficult to obtain agglomeratcs in accordance with this method which have sufficient reduction strength to enable them to be used in blast furnaces.

In accordance with an earlier method developed by applicant (Swedish Patent No. 304,767 and Belgian Patent No. 698,604) oxidic material is rolled at a temperature as low as 200C, whereupon satisfactory agglomerates are obtained and the said difficulties completely removed. Thus, it is possible to obtain an agglomerated material which can be handled, transported, stored and subjected to further metallurgical processes in such operations as sintering in open furnaces or on conveyor belts, treatment in rotary furnaces, shaft furnaces,

. flame furnaces, electric smelt furnaces etc. The agglomerates obtained when practising the method of the said patents can also be intermixed with a charge which is to be suction sintered, for example.

In many instances, however, it is favourable from a practical point of view to be able to work without heating the material or by heating to moderate temperatures of below about C. It has now been surprisingly discovered that when using certain materials and under certain conditions it is possible to cold roll iron raw materials or iron raw materials heated to moderate temperatures, with extra ordinarily good results.

If an oxidic iron material is to be compressed between rollers at temperatures below 100C it is necessary, because of the high friction between the grains, to use a compacting pressure of such magnitude that rolling is practically impossible to carry out technically.

This problem can be solved by rolling the oxidic iron material at low temperatures in the presence of a certain quantity of a lubricant, preferably water. In this way the compacting pressure at low temperatures can be lowered considerably while maintaining the desired degree of compression. If the lubricant still remains on the grain surfaces upon completion of the compressing operation, the agglomerate will obtain a low mechanical strength, since the grains are still able to slide against each other. It is therefor important to use a lubricant which can be removed by vapourization subsequent to compressing the material. Water is a suitable lubricant in this connection. The water can be vaporized by subsequently drying the compressed material, but this means that the material must be transported and handled prior to and during the drying in a relatively weak state, and there is a risk of the agglomerated material disintergrating in the process. Afterdrying also constitutes an expensive complication of the process.

In accordance with the present invention, material is agglomerated at a temperature of normally below 100C between two smooth or grooved rolls subsequent to adding a lubricant, suitable water, in such quantities that an effective friction reducing effect is obtained, and that the quantity of lubricant added reaches at most to the quantity which can be substantially evaporated by means of the heat produced by the friction. Any friction reducing agent can be used of course, although water is to be preferred since the material is not contaminated thereby. If any agent other than water is used it must be removed by evaporation. Examples of such agents are volatile organic liquids, preferably hydrocarbons. Mixtures of water and organic liquids and aqueous solutions containing surface tension reducing agents can also-be used to advantage. It is, of course, also possible to compress a material heated to a specific elevated temperature, since in such a case larger quantities of lubricant can be removed by means of the heat developed big friction and, moreover, the surface tension of the lubricant falls and the friction reducing effect increases, whereby a higher degree of compression at a given pressing pressure can be obtained.

As previously mentioned, roasted goods from the roasting of flotation pyrites have long caused great difficulties when being handled, stored, transported and, above all, when being sintered. When practicing the method of the invention, strong small agglomerates are obtained, which in addition to good physical properties with regard to handling, storage and transport also show particularly good results when being sintered, both with respect to the capacity of the sintering process and the quality of the sintered product.

Another example of a process in which material agglomerated in accordance with the present invention can be used to particular advantage is the rotary furnace process (Dored), in which ironoxide material, normally without being agglomerated beforehand, is reduced in a rotary furnace to pig iron in a slag-coke layer floating above the pig iron bath. When charging very fine raw material to the furnace in this process disadvantages are encountered in the form of powder clouds and difficulties in distributing the material over the surface of the bed. In order to reduce consumption of coke and to increase the capacity of the reduction furnace the iron oxide material is normally dried and pre-heated with hot waste gases taken from said furnace. By rolling the fine material in accordance with the present invention a coarse grained, dry material can be obtained, which is more suited for charging to the reduction furnace.

The compressed material has also been found particularly suitable as a starting material when manufacturing sponge iron in both rotary furnaces and floating bed furnaces.

If a more coarse product is desired, the material can be screened after being compressed, whereafter the finer grain material is returned for renewed agglomeration. When proceding in this manner a more compact agglomerate of higher mechanical strength is obtained. This is particularly so if the screening is adapted so that the returned material together with the untreated material obtains a screening curve whereby the mixture obtains the least possible number of pores and the highest degree of compactness. Grain size distribution should, therefor, conform to the known Fuller-curve." A grain size distribution of this nature can be provided by adding to the input material one or more grain fractions screened from the rolled product. The material being compressed can also be assured the suitable grain size distribution by mixing different starting materials with various grain size distribution. Since gases, such as air and steam, trapped in the compressed material often cause crack formation and non-uniform roll operation, it is particularly advantageous when rolling in accordance with the present invention to associate the grain size distribution to the Fuller-curve," whereby smaller quantities of gases are entrapped when compressing and a considerably stronger product is obtained.

If the number of pores is reduced in the aforedescribed manner the supply of material to the rolls is also simplified, since a considerably smaller quantity of gas (air and steam) need be displaced during the compression operation. The displaced gas, flowing in counter current to the direction in which the material is charged, has previously often necessitated the use of feed screws in order that the fine grain material can be fed to a rolling operation. Consequently, if the quantity of gas which must be displaced during a compressing operation can be cut low and, furthermore, when correcting the grain size distribution in accordance with the aforegoing a coarser, readily compressed more compact material is used it has been found possible to compress the material without the use of special feed screws. This is of particular importance with regard to the present process since a certain quantity of water is supplied which must be removed in the form of steam during the compressing operation.

The method has successfully been applied on both hematitic iron oxide obtained from oxidizing fluidized roasting of pyrites and on iron oxide obtained from magnetite yielding roasting operations. The method has been found particularly suited for use with a magnetic material since the agglomerate has been found stronger and more water-resistant than corresponding agglomerates of hematitic iron oxide. The reason why the hematitic material shows a tendency to decompose in water is not known, although Applicants investigations show that this may be due to the presence of a certain quantity of water-soluble salts on the grain surface, which are dissolved in the presence of moisture. One reason contributing to the difference may. also be that the surface of the magnetite grains are oxidized to a certain extent, whereupon the occurrence of crystal conversion contributes to. a stronger bond between the grains. It is namely known that magnetite, particularly newly formed magnetite, is oxidized at room temperature to a certain extent. It has also been discovered that fresh magnetite obtained from a fluidized roasting process carried out in accordance with Swedish Patent No. 204,002 provides a splended agglomerate. The tendency of the magnetite to oxidize on the grain surfaces at low temperatures can be used for a low temperature hardening of the dry agglomerates. A five-fold increase in compression strength in comparison with material not subjected to heat treatment is obtained at a temperature increase of about 250C.

As previously mentioned, the method of the invention is concerned with the compression of the material between smooth or grooved rolls. If the rolls become uneven as a result of wear they can be turned or ground during operation of the plant. lf, furthermore, the rolls are given a large barrel thickness they can be turned or ground down a large number of times. The rolls can be protected from attack by coating the same with, for example, graphite of CaO or with a coating of ceramic material based on, for example, aluminum oxide or carborandum. A certain amount of wear will not affect the result of the process to any great extent, since, as previously mentioned, the rolling operation is not intended to produce briquettes of a definite shape, but that a certain quantity of the fine material can be tolerated in contradistinction to the case when briquetting, for example. it is also suitable to spray the rolls with a lubricant in order to prevent the material from adhering to the rolls.

When water is used as a lubricant, the ingoing material is prepared so that it obtains a temperature below 100C and a water content of 26%, preferably 2-4%. The water can be added in the form of liquid and/or steam. In the latter case, the steam condenses in the material and provides the desired quantity of water therein. If prior to being compressed the material has a temperature in excess of 100C, it is cooled by spraying water thereon, while if the temperature is low it can be raised by heating with steam. Adjustment of temperature and water content can be effected so that a practically dry material is obtained subsequent to the compressing operation. The roll pressure needed to effect the compression of the material naturally depends on the dimension of the rolls and the physical properties of the starting material. For example, iron oxide taken from a pyrite roasting operation can be rolled with good results using a roll diameter of 1 meter and a roll pressure of 5 l0 MN/m (t/cm).

The following example illustrate a preferred embodiment of the invention and also show the result of a comparison test in which a lubricant was not used.

EXAMPLE The test was carried out with a compression plant provided with double rolls having a diameter of 1 m and a roll width of 138 mm. A hydraulic pressure of 6 9 MN/m (t/cm) roll length was applied between the rolls.

The starting material was magnetitic iron oxide obtained from a magnetite yielding roasting of pyrite in accordance with the Swedish Patent No. 204.002. The ingoing material had a screen analyses of 79% beneath 0.063 mm and 99.9% beneath 0.125 mm, a volume weight in an uncompacted state of 0.9 kg/cm and a bulk volume weight of 1.1 kg/cm. The material was fed by a vertical screw. The roll surfaces were sprayed with silicon oil.

TEST

Water content 2,9 0 0 4 Mixing temp. C 55 60 140 140 ca20 Roll cake thickness mm 6,5-7,0 8,0 8,0 7,0 Press pressure t/cm 7,5 9 7,2 volume weight g/cm 2,23 2,32 2,27 compression relationship 2,45 2,5 2,50 remaining water content 0,3 0 0 3,0 Screen analyses no or slight agglomera- 1,0 mm 73,0 41,2 tion 79,0 1,0-0,2 12,9 7,8 8,2 0,2 0,125 3,7 0,2 4,3 0,125-0,090 4,5 0,5 0,7 0,090-0, 63 3,2 9,5 3,0 0.063-0,050 1,1 16,8 2,4 0,050-0,040 0,655 19,5 1.8

As will be seen from the table, test 1 was effected (in accordance with the present invention) with a material having a water content of 2.9%, at a temperature of 55-60C and at a pressure of 7.5 t/cm, the compressed material obtaining a residual water content of 0.3%. In test 2 a dry material was compressed by way of comparison at C and at a pressure of 15 t/cm. in test 2a the same material was compressed in the same manner as in test 2 and at the same temperature but at a pressure of only 9 t/cm. In test 3 a cold material (approximately 20C) was compressed with a water content of 4% at a pressure of 7.2 t/cm, the compressed material obtaining a residual water content of 3.0%.

It can be seen from the screen analyses shown in the table that both in test 1 and test 3 good compression was obtained. An examination of the compression strength and the wear resistance of the material showed, however, that said material from test 1 had very high strength properties while the strength of the material obtained from test 3 was unsatisfactory. Furthermore, the material obtained from test 3 showed a tendency to disintergrate when moistened. The- 'agglomerates obtained from test I remained intact when wetted and the maximum amount of water absorbed was low, beneath approximately 68% with saturation (approximately 20-25% of a nonagglomerated material). Complementary laboratory tests have shown that the difference in strength properties and water sensitivity of material from test 1 and test 3 respectively can not be described to the higher residual water content of test 3.

Test 2 and 2a show that it is not possible-to obtain a satisfactory agglomerate without the addition of a lubricant even though the compressing pressure is doubled. ln tests 1, 2 and 3 the obtained cake is decomposed into a material presenting the screening analyses given in the table without it being necessary to employ any special crushing operations.

What is claimed is:

l. A process of agglomerating fine-grained iron oxide material into a coarse-grained product which lends itself to easy handling and-transportation and which is suitable for further metallurgical treatment comprising adding as the sole lubricating agent water to said material in an amount of 16% by weight of said material controlling the temperature of the moistened material to within the range of 55100C, and agglomerating the material by rolling and compaction of same between substantially smooth rolls, under pressure at least sufficient that the heat of friction developed in the material is such as to effect vaporization of the water theretofore added, and thereby produce a dry strong agglomerate, said water being vaporized by the friction heat developed during the rolling.

2. The process of claim 1 wherein the temperature is raised by injecting steam into the cold material to be compacted, where the steam condenses to the desired water content.

. 3. The process of claim 1 wherein said fine-grained iron oxide material is magnetite.

4. The process of claim 3 wherein the agglomerated material is subsequently hardened at a temperature of 200300C. 

1. A PROCESS OF AGGLOMERATING FINE-GRAINED IRON OXIDE MATERIAL INTO A COARSE-GRAINED PRODUCT WHICH LENDS ITSELF TO EASY HANDLING AND TRANSPORTATION AND WHICH IS SUITABLE FOR FURTHER METALLURGICAL TREATMENT COMPRISING ADDING AS THE SOLE LUBRICATING AGENT WATER TO SAID MATERIAL IN AN AMOUNT OF 1-6% BY WEIGHT OF SAID MATERIAL CONTROLLING THE TEMPERATURE OF THE MOISTENED MATERIAL TO WITHIN THE RANGE OF 55*-100*C, AND AGGLOMERATING THE MATERIAL BY ROLLING AND COMPACTION OF SAME BETWEEN SUBSTANTIALLY SMOOTH ROLLS, UNDER PRESSURE AT LEAST SUFFICIENT THAT THE HEAT OF FRICTION DEVELOPED IN THE MATERIAL IS SUCH AS TO EFFECT VAPORIZATION OF THE WATER THERETOFORE ADDED, AND THEREBY PRODUCE A DRY STRONG AGGLOMERAE, SAID WATER BEING VAPORIZED BY THE FRICTION HEAT DEVELOPED DURING THE ROLLING.
 2. The process of claim 1 wherein the temperature is raised by injecting steam into the cold material to be compacted, where the steam condenses to the desired water content.
 3. The process of claim 1 wherein said fine-grained iron oxide material is magnetite.
 4. The process of claim 3 wherein the agglomerated material is subsequently hardened at a temperature of 200*-300*C. 